The present invention relates to a controllable time-interval signal generating apparatus for an electronic timer.
Heretofore, a relaxation oscillator has frequently been employed as the time-interval signal generating apparatus for an electronic timer. In such relaxation oscillators a variable resistor and a capacitor form a relaxation oscillator circuit. The frequency of a signal generated by the relaxation oscillator circuit is then stepped down by a frequency divider and the number of pulses is counted by a preset counter. The preset counter, which has been set beforehand, is adapted to generate a time-interval signal when a predetermined number of pulses has been counted.
In this type of conventional time-interval signal generating apparatus, a time interval is obtained which is related to the resistance value and the electrostatic capacitance of the relaxation oscillator circuit. Therefore, the time interval is adjusted by varying the resistance value of the variable resistor. Although such a conventional time-interval signal generating apparatus is simple in construction and easy to handle, the oscillation frequency may vary undesirably with changes in the resistance value of the resistor due to a temperature or humidity change or with a change in supply voltage. Such changes, in turn, cause variations in the time interval when the signal is generated.
Further, such conventional time-interval signal generators which use the same circuit may produce differences in time intervals due to variations in the actual resistance or electrostatic capacitance of the components used in the circuit from the nominal values for those components. For this reason, in order to obtain an accurate time interval signal, it is necessary to hold the supply voltage constant and to employ a variable resistor and a capacitor having a high precision in value. In addition, a fine-adjustment circuit for correcting the above-described variations is required, which is an additional disadvantage.
Accordingly, a primary object of the present invention is to provide a time-interval signal generating apparatus capable of producing an accurate time interval signal which is independent of fluctuations in control power supply voltage, or variations in the characteristics of the circuit elements, or environmental changes.